PartFile: atomic-rename SavePartFile to cut per-tick disk I/O - #670

…tching category (amule-project#670)

Typing in the transfer-window text filter froze the GUI for seconds on a
large queue -- ~10 s per keystroke with 10k downloads, with keystrokes
stacking up (issue amule-project#669).

SetFilterText() re-evaluated every model item and called ShowFile(file,
visible) for each one. Every row that had to disappear went through
RemoveItemData(), which does a full RebuildRowIndex() plus a
RefreshFromRow() -- O(n) per removal, so O(n^2) for the pass. The
shared-files filter is instant because it does the opposite: it clears the
model and re-appends the passing items, finishing with a single
FinishBulkLoad().

Give the download list the same shape. RebuildVisibleList() clears, appends
every item passing the current category + text filter, and finishes with one
FinishBulkLoad() (one SetItemCount, one index rebuild, one sort) -- O(n log n)
and a single repaint. ShowFileList(), the filter and ChangeCategory() all
route through it, so the identical O(n^2) in the category switch is fixed too
and the three paths no longer duplicate the visibility loop.
ShowFilesCount(diff) now delegates to SetFilesCount(count) so the rebuild can
set the count absolutely without duplicating the label update.

The two lists had also grown a filter each: identical m_filterText members,
identical SetFilterText() bodies and byte-identical PassesTextFilter()
implementations differing only in the parameter type. That now lives once in
CMuleVirtualListCtrl -- m_filterText, SetFilterText() and a string-based
MatchesFilter(), plus a RebuildFilteredView() hook each list overrides with
its own rebuild. The base stays agnostic about what its rows represent.

A clear-and-re-append rebuild renumbers every row, and the virtual control
tracks selection and focus by row index, so both would be lost -- the
incremental path kept them for free by leaving surviving rows untouched.
CMuleVirtualListCtrl grows SaveSelection()/RestoreSelection() for this;
SortList() already did the same save-and-reapply inline and now shares them,
as does CSharedFilesCtrl::ShowFileList(), which fixes the pre-existing loss
of selection when editing the shared-files filter.