Allow caching imagery by bounding box

[jidanni]

Currently one must zoom in and out, and sweep left, right, up, and down, to make sure an area of e.g., Bing imagery will be cached and thus available later offline.

There are better ways, e.g.,
​https://www.alpinequest.net/en/help/v2/maps/on-demand-store-area

In fact one should be able to define a box and hit 'cache imagery', just like one currently can do for vector data.
In the preferences dialogs one does have control over what do delete. But acquiring imagery is still done only via hand.

Revision:13265

[stoecker]

Closed as duplicate of #15591.