Integrate support for multiple simultaneous WMS layers into msui client

[ReimarBauer]

Original report by Marc R. (Bitbucket: wxmetvis, ).

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Currently the msui can only display one WMS image at a time. Similar to other WMS clients, an overlay of multiple (transparent) maps should be possible.

This could be realized by implementing:

• support for multiple WMSControlWidgets
• these WMSControlWidgets should (optionally) synchronise level, init time, valid time with a "master" control
• multiple images may need to be combined into one image for display with matplotlib -- maybe this has changed, otherwise PIL (Python Imaging Library) should provide functions

[ReimarBauer]

Original comment by Marc R. (Bitbucket: wxmetvis, ).

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We already have code for this, needs to be reviewed and merged.