feat: use the application iconfeature/qaction-menus

The icon was committed in a previous session and referenced nowhere: no
qrc, no .desktop entry, no setWindowIcon. It is wired up now, as a window
icon, a desktop entry, and install rules placing both into hicolor and
share/applications.

resources.qrc belongs to the executable rather than to qtmaildir_lib. A
qrc compiled into a static library registers itself from a global
initialiser, and the linker discards that object because nothing
references it: the build succeeded, qInitResources_resources() was
present in the .a, and QFile::exists(":/icons/qtmaildir.svg") still
returned false at runtime. Verified loading at 16, 32 and 64 pixels after
the move.

Toolbar and menu actions take icons from the system theme by their
standard names, so they match the rest of the desktop rather than
shipping bespoke art. A theme lacking one leaves that action as text,
which still works.

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