Added queue's total time & case insensitive sorting; ignore empty search string

[embhd]

• Added queue's total time to panel header (closes Cumulative time for Queue #47)
• Enable case insensitive sorting of database entries while browsing
• Since an empty search string always seems to just time out, it is now simply ignored

Unfortunately, I had to "pre-merge" the original three branches, since I had to make some final changes for everything to work as expected (and to be auto-mergeable), c.f. 71ec2cd.

Additional changes:

• Removed compiler warning "comparison of constant -1 with expression of type 'enum mpd_cmd_ids' is always false"
• Fixed main table layout in case of an empty queue and search result, respectively
• Split-up of a few very long code lines
• Update link behind navbar's "Browse Database" after every page reload (i.e. after following a link)
• Make text input in "Save Queue" behave like the one in "Add Stream"
• Some clean-up