Merge branch 'terencehill/menu_optimization' into 'master'

Merge branch 'terencehill/menu_optimization' into 'master'

Menu optimization

List of demos, screenshots, music tracks and player models (current player model image too) are now loaded on display, previously both on start (2 times due to a bug) and on display

List of cvars and keybinds are now loaded only on display (previously only on start)

Implementation changes:
* Removed an unnecessary showNotify call for every item when building up the menu (caused to load many file lists twice)
* Added SUPER(XonoticCvarsDialog).showNotify(me); in the implementation of showNotify for dialogs so that showNotify gets called for every item in the dialog (e.g. for the cvarlist in the "Advanced Settings" dialog)

See merge request !263