Update statusbar on initial cell selection

Do not update cell amount in statusbar on hover

Increment copyright year

Increment copyright year

Update status bar on select all

Update formatting and section headers in README.md

Revise TreeSheets description in README

Updated the description of TreeSheets to emphasize its features as an open-source data organizer and hierarchical spreadsheet.

Customize toolbar appearance

Do not draw border and keep same background as toolbar

Use plain background color for wxAuiToolbar

Also remember thin-selected cells