OSC: Rework handling of right-click in the device/channel tree

The right-click menu will now only appear if performed on a device or
channel. The underlying channel or device will be highlighted, which
forces the "color settings" sub-menu to consider the channel that was
right-clicked instead of the last one selected.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Use standard Cancel/Apply buttons for the Impulse Generator dialog

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
OSC: Add a dialog to specify trigger settings

This dialog can be opened by doing a right-click on a device name.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>