app: Remove old main window code
Move application translations to the appropriate directory

Issue: #26
Refresh translations at build time

Issue: #26
Move desktop entry translations with others

The source file is moved to src/app though.

Issue: #26
Move plugins desktop entry translations

Issue: #26
Move translations for the other plugins as well

These plugins are not currently being developed but we have old
translations and we want to move them to the new directory structure.

Issue: #26
background: Replace wallpapers model with a simplified version

The new BackgroundsModel only handle backgrounds (that is pictures
located in $XDG_DATA_DIRS/backgrounds or custom paths) and doesn't
handle KDE wallpapers at all.

The model doesn't load backgrounds at startup result in a more
efficient startup of the preferences panel.

Issue: #29
background: Say hello to ColorsModel

Model with colors to be used for solid and gradient backgrounds
configuration.

Issue: #19
background: Bump QML API version to 1.0

Issue: #19