Harmonize input context selection

Harmonize input context selection

Input context selection works differently across platforms. On some
platforms it is not possible to request a specific context at all
(e.g. Wayland). This will be unified, depending on the environment
variable "QT_IM_MODULE", you will get:
- null:  default (platform) context, if defined (otherwise no context)
- empty: no context
- set:   set one, if it exists and is valid (otherwise no context)

[ChangeLog][Platform Specific Changes] Haromnized input context selection.
QT_IM_MODULE environment variable will be taken into account.

Change-Id: Ic8f826fbc6ace25941cd19b9b086943e848fbe01
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Nedim Hadzic <nedim.hadzic@pelagicore.com>