control: Fix PHP warning (Array to string conversion)

PHP Notice:  Array to string conversion in control.php on line 255

Signed-off-by: Stefan Weil <sw@weilnetz.de>
control: Don't add a non-existing window to the database

If opening of a new window fails (problems with viewer), there is no
new window id and nothing should be added to the database.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
control: Rename variable $dbcon to $db

The shorter variant is already used in DBConnector.class.php.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
palma: Only show one icon in user list entries

The device icon is sufficient.
Remove the user icon which does not contain any information.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
palma: Show user list on top of item list

Signed-off-by: Stefan Weil <sw@weilnetz.de>
palma: Fix new html errors

The html code now passes W3C validator without warnings or errors.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
control: Make deletion of all windows more robust

Clean any remaining window entries in the database.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
css: Improve position and width of help window

Signed-off-by: Stefan Weil <sw@weilnetz.de>
upload: Add missing html code to error.html

Error messages are created on the fly in a file called error.html.
The error text can contain UTF-8 characters, so some html code is
needed to get a correct display.

Signed-off-by: Stefan Weil <sw@weilnetz.de>