Linux: handle dark system themes nicely

yuzu's default theme doesn't specify everything, which is fine for
windows, but in linux anything unspecified is set to the users theme.

Symptoms of this are that a linux user with a dark theme won't think
to change the theme to a dark theme when first using yuzu

Idea here is to try and support arbitrary themes on linux.

preliminary work on a "default_dark" theme, used only as overlay
for any themes that are measured to be dark mode.

Other work done:

FreeDesktop standard icon names:
plus -> list-add
delete refresh, we use view-refresh

remove duplicated icons for qdarkstyle_midnight_blue
referencing icon aliases in the qrc files is the way to go

Note:
Dynamic style changing doesn't appear to work with AppImage