Get Leaflet map size from QGIS canvas size

When you click the dialog's Get Layers button, it also pulls over the
QGIS map canvas dimensions. This could be implemented as an extra button
in the UI, but I could not get that to work.
Fixed SimpleLine styling

SimpleLine styling caused a plugin crash.
opacity_raster CSS bug

The CSS was missing two closing }s if opacity_raster==True.
Tidy pop-up

Tidier layout for pop-ups (remove header row, make field name cells THs)
Correct layer order

Fix for #60
Add basemap to Layer Control

Feature request #70
Separate getMapSize from getLayers

New UI button to allow independent retrieval of layers and map size.
Merge pull request #69 from tomchadwin/master

Get Leaflet map size from QGIS canvas size