webaccess: various small improvements

[jpue]

by-product of #1879

• RGB matrix: allow stopping RGB Matrix functions (previously not possible since the webaccess matrix slider was limited to a minimum value of 1 instead of 0)
• RGB matrix: update the algorithm list in webaccess when changing the algorithm (both locally and via the web interface) using custom buttons
• webaccess: fix a JavaScript error visible in the browser’s developer tools when moving a slider as sliders were always incorrectly treated as knobs
• simpledesk: check parameters to avoid a NULL-pointer dereference and correct generated HTML code
• keypad: fix WebSocket error handling
• remove unnecessary type="text/javascript" attribute from script tags
• simplify logic statements within if statements

Furthermore, I can add the following changes (here or in a separate PR), if desired:

• HTML-encode all user-defined strings embedded in the HTML code to prevent HTML syntax errors and injection attacks; however, this means losing the ability to customise the webaccess UI beyond the official options using the HTML tags <b>, <i> and <br>
• split the DMX keypad (currently a standalone, static HTML file) into a separate CSS and JS file and integrate the HTML code as a static function into C++ code source of webaccess/simpledesk, enabling the use of some common code as well as the translation of the DMX keypad

[coveralls]

coverage: 34.231% (-0.001%) from 34.232%
when pulling 25d99e0 on jpue:webaccess_v4
into 9e87b0d on mcallegari:master.

[mcallegari]

Thanks 👍

Just one little note.
Sometimes I prefer "condition == false" instead of "!condition" because it is more readable.
If I don't see the '!' then I might read the opposite meaning of a condition.
Maybe it's just me getting old though...