New Insert Table dialog

Added a new dialog to define the size, alignment and content
of a table.

Issue #51
Add method insertTable to MarkdownManipulator

Add new method insertTable() to MarkdownManipulator class. This method
inserts a markdown table defined with the TableToolDialog.

Issue #51
New menu item "Insert Table..."

Add new action "Insert Table..." to edit menu. It shows the
TableToolDialog and uses the MarkdownManipulator class to insert a table
into the markdown document.

Issue #51
Merge branch 'feature/table-tool-#51' into develop