allow input methods to be enabled

[totaam]

Currently, the server will try to disable all input methods to try avoid problems by setting the following environment variables:

DISABLE_IMSETTINGS=true
GTK_IM_MODULE=xim
QT_IM_MODULE=xim
IMSETTINGS_MODULE=none
XMODIFIERS=""

But some users (see this ML discussion) may want to enable input methods without knowing about all those values.
For example for ibus:

GTK_IM_MODULE=ibus
QT_IM_MODULE=ibus
XMODIFIERS="@im=ibus"

We should provide a better way of enabling this.

We need to:

• figure out what normally sets those values (a session script somewhere?)
• does any of it need dbus or other dependencies?
• does it interfere when running multiple sessions
• what is the best config / command-line switch? (input-methods=[none|ibus|...)

[totaam]

Done in r7272.

Adds the input-method switch. Options:

• none
  Disable input methods completely and prevent it from interfering with keyboard input. This is the default.
• keep
  Keeps the environment unchanged. You are responsible for ensuring it is correct.
• xim
  Enables the X Input Method.
• IBus
  Enables the Intelligent Input Bus.
• SCIM
  Enables the Smart Common Input Method.
• uim
  Enables the Universal Input Method.

[totaam]

Was done for milestone 0.14

[totaam]

See also: GTK apps do not integrate with the Windows input..