app: Use QQmlApplicationEngine

Closes #20
Deferred preflet item creation

Create QML component as soon as the model is populated and create items
the first time a preflet is requested.

Closes #21
app: Simplify layout a little bit

Use ColumnLayout as a top level item.
app: Make it flickable

Turn the main page into a flickable item.

Closes #22
app: Remove categories model
Plugin manager

With a plugin manager we only search and load plugins once and create
models for categories based on the list of plugins we found.

We also change the plugin API and move metadata to desktop entries that
will also be used to run the preflets from the desktop environment.

Closes #24
app: Load preflet passed to command line

Closes #27
Say hello to the keyboard panel

Initial keyboard configuration, shortcuts are not yet implemented.

Closes #23
keyboard: Make group boxes checkable

Instead of using a switch we make group boxes checkable and let QtQuick
Controls handle this.
Pull translations from Transifex
Fix errors in desktop entry

Fix errors after validation.
Update translations
network: Add translations
keyboard: Remove leftovers
keyboard: Save enabled settings
background: Set type after color or wallpaper
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