Make joining lines in Visual Mode with J work when the selection is in a downwards direction, and fix counted-ness of line joins (it was joining one too many) and make cursor positioning match Vim's (should place the cursor before the first non-whitespace char of the last line joined).

If the count would take us past the number of lines in the document in Normal mode, do nothing.

Don't crash if there are only empty lines to join.

Also, entirely unrelatedly, fix some annoying warnings in KateViModeBase::switchView(...).