Enable input context selection

It was not possible to use a different input context than the platform
context. This has been unified across major platforms, depending on the
environment variable "QT_IM_MODULE", the following context is selected:
- null:  default platform context (Wayland in this case)
- empty: no context
- set:   set one, if it exists and is valid (otherwise no context)

Change-Id: I07c6fb339c434f99fc6e092a2e18f00600daa3bc
Reviewed-by: Nedim Hadzic <nedim.hadzic@pelagicore.com>
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>