client: Add Highlight Rules tooltips, UI polish

client: Add Highlight Rules tooltips, UI polish

Add tooltips to every element of Highlight/Highlight Ignore tables,
and to the Local Highlight table.

Update the "Channel" tooltip to describe the non-regular-expression
approach used now.

Rename "Import" to "Import Local", add tooltip to better describe
purpose.

Move "Highlight Ignore Rules" table into QGroupBox to place at even
height with "Highlight Rules" box, making it less jarring to switch
between the two.