save_tabs function is way too aggressive #1633

On current master (), the save_tabs function is being called way too much.
When PS1 is set to update the title, save_tabs is called every time Enter is pressed in guake.

To reproduce:
Add this to .bashrc, or just run this snippet in a guake shell.

# If this is an xterm set the title
case "$TERM" in
xterm*|rxvt*)
    PS1="\[\e]0;\W\a\]$PS1"
    ;;
*)
    ;;
esac

Press enter in guake terminal for about a second:
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561134
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json
mom, I've been called: on_terminal_title_changed <function Guake.on_terminal_title_changed at 0x7fd9a0196ea0> at 1566561135
Guake tabs saved to /home/user/.config/guake/session.json