Conflict with other extrensions using Ctrl+M shortcut

[matuscak]

There are other extensions which are using the same Ctrl+M shortcut, e.g. KeePassRDP. When KeeAgent extension is installed, this shortcut does not work any more in them.

I do not know KeePass programming internals but I believe there should be some correct way how to implement this logic:

1. KeeAgent receives "Ctrl+M pressed" keyboard event
2. KeeAgent recognizes that currently selected item is not "KeeAgent enabled" so it knows that received keyboard event was not addressed to itself. Therefore it does not "consume" the event (or returns it back to the event queue) so other plugins in the chain could receive it and process it accordingly.

[dlech]

Related: #292