Widgets: allow external RT backends to include configuration dialogs

[pedrolcl]

This comes from SourceForge Ticket 43.

Currently, some of the Drumstick::RT plugins have configuration dialogs residing in Drumstick::Widgets. This library also has functions to query and show these dialogs:

drumstick/library/include/drumstick/configurationdialogs.h

Lines 40 to 43 in 93a41f2

	bool DRUMSTICK_EXPORT inputDriverIsConfigurable(const QString driver);
	bool DRUMSTICK_EXPORT outputDriverIsConfigurable(const QString driver);
	bool DRUMSTICK_EXPORT configureInputDriver(const QString driver, QWidget* parent = nullptr);
	bool DRUMSTICK_EXPORT configureOutputDriver(const QString driver, QWidget* parent = nullptr);

External RT plugins having their own configuration dialogs could be developed and distributed out of tree. The same rules and functions for configuration dialogs should apply to them.

For instance, you can already use an out-of-tree template plugin that includes (fake) configuration dialogs.

Another out-of-tree drumstick backend is the drumstick-rt-fluidlite which also includes a configuration dialog:

Implementation in r662 or a7acb2b will be available in v2.7.0

[pedrolcl]

Drumstick v2.7.0 has been released.