Fix for https://github.com/hluk/CopyQ/issues/1630

Fix for https://github.com/hluk/CopyQ/issues/1628

Breaks restoring geometry on Wayland, but it did not work well anyway.

Fixes #1624

Changing theme crashes the app in some cases with the default
"windowsvista" Qt style.

This fix avoids calling decoratePreview in quick succession.

Fixes #1621

Fixes #1085

Getting mouse position on Wayland might be unsupported.

Fixes #1558

Currently translated at 26.6% (166 of 623 strings)

Currently translated at 0.6% (4 of 623 strings)

Adds "style" option and styles() function.

This allows updating application style/theme in case system theme
changes without notifying the application.

Uses undocumented QEvent::ThemeChange to detect theme changes.