Miscellaneous improvements

[AlisterH]

1. Remove either the OK or cancel button, as there is no difference between them
2. Main QGIS window should not be raised above the plugin window after displaying an error message.

EDIT: both done

[AlisterH]

3. Need refresh button for the list of layers to choose extent layer.
4. It currently only lists layers that are turned on, but it would be better if it listed all vector layers (and even better if it allowed to select a file that isn't loaded into QGIS).

Edit: done/no longer needed

[AlisterH]

1. Remove either the OK or cancel button, as there is no difference between them

Done

2. Main QGIS window should not be raised above the plugin window after displaying an error message.

Done

3. Need refresh button for the list of layers to choose extent layer.

No longer needed as I have it refreshing automatically.

4. It currently only lists layers that are turned on, but it would be better if it listed all vector layers (and even better if it allowed to select a file that isn't loaded into QGIS).

Done

[AlisterH]

5. It would be good to be able to use more than one vector layer.
6. It would be good to inform the user if the input layer is empty, instead of creating empty coverage layers.

[AlisterH]

7. It would be good to support raster input layers.

These features could be implemented with a processing model, if we have a processing algorithm (#8)