Reimplement keyboard layout selection

Moved keyboard layouts into locale specific directories and removed
locale identifier from the layout file name. The FolderListModel now
tracks the locale directories instead of layout file names. The default
locale is now initialized based on current default locale (e.g. QLocale()).
The locale selection also accepts partial locale names (e.g. language_*).

The keyboard layout selection allows to override only those layout types
that are different from the en_GB layout. For simple country variants
such en_US only the specific layouts needs to be customized (e.g. symbols
layout), while using default for the other layouts.

The keyboard layout selection is now triggered manually in the Keyboard.qml
instead of using bindings. With binding the keyboard layout could be changed
multiple times before the final layout was found.

There are now three kinds of numeric layouts, one for each exclusive numeric
modes of the input method hints; digits, dialpad and numbers. The numbers
layout is now dedicated for ImhFormattedNumbersOnly mode and it contains
some extra math symbols.

Added new parameter customLayoutsOnly for Keyboard.changeInputLanguage()
function. When set to true, the list of possible languages is limited to
those providing the specific layout type.

Task number: QTRD-2823
Task number: QTRD-2898
Task number: QTRD-2899

Change-Id: I9b0e5cf4110a58313b7320af571aca899c55749c
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>