nitpicker: defer hover changes during drag operations

[nfeske]

When resizing a window with a size constraint using the motif decorator and depressing the mouse button while the pointer is outside the resize handle, the resize handle stays in hovered state.

During the analysis together with @chelmuth, we found that nitpicker updates its internal _hover state too eagerly, which indirectly leads to the masking of a leave event to the decorator. This issue may be related to the sporadic "jumping window" effect that sometimes occurs in Sculpt.

[cproc]

@nfeske: with your latest commit I could not reproduce the jumping window or resize handle hovered state anymore.

[nfeske]

@cproc, thank you for the confirmation. :-)