This giant ugly blob of interconnected changes PR:

• Replaces text with icons in the abilities menu and adds ability descriptions;
• Replaces "end turn", "menu" and "info" buttons with icons too;
• Removes the "deselect" button;
• Hides the "End Turn" button during enemy's turn;
• Adds a battle exit & campaign exit confirmation dialogs (and enum BattleType { Skirmish, CampaignNode } to distinguish them).
• Adds popup screens and renames screen::Transition to screen::StackCommand;
• Allows grouping related widgets into panels;
• Allows exiting from the ability mode by clicking on the ability icon again;
• Adds a black frame, an is_active property and custom color highlighting to zgui::Button;
• Adds new zgui widgets: ColoredRect, LayersLayout;
• ... aaand does a bunch of smaller tweaks and changes.

All icons:

Battle screenshot showing icon buttons, ability descriptions, agent info grouped into one panel:

Example of widgets grouping & grayed-out buttons in campaign menu:

Example of confirmation dialog:

Depends on ozkriff/zemeroth_assets#7

Closes #150 ("Gray-out inactive buttons")
Closes #276 ("Replace text with icons in abilities menu")
Closes #387 ("The player shouldn't be able to restart a battle in a campaign")
Closes #560 ("Exit confirmation dialogs")
Closes #419 ("zgui: Make buttons visually distinctive from labels")