Add ability to (block) (un)comment in Beanshell/SQL consoles

Here it is! Now we can finally comment and uncomment text in the Beanshell and SQL consoles without having to type `--` or `//` for every single line.

I implemented the same commenting behavior I found in other major programs such as IntelliJ, Netbeans and Eclipse. In particular, toggling the comment function gives the following behavior:
```mysql
----CREATE TABLE ...                         --CREATE TABLE ...
--CREATE TABLE ...      <- becomes ->      CREATE TABLE ...
----CREATE TABLE ...                         --CREATE TABLE ...
```

#### Keyboard Shortcuts
* <kbd>ALT</kbd>+<kbd>c</kbd> - Comment/Uncomment lines. I.e., prepend a `--` (SQL) or a `//` (Beanshell) at the beginning of each line containing selected characters (or at the beginning of the current line if the selection is emtpy), or remove them if every line in the selection is commented.
* <kbd>ALT</kbd>+<kbd>SHIFT</kbd>+<kbd>c</kbd> - Block comment. I.e., surround the selection with `/*` and `*/` (both SQL and Beanshell).

Fixes #564.