Add additional {{ variables }} to HTML Widget, to convey widget dimensions

The widget is currently capable of passing the following variables:

• {{device}} – current device or group, you can access its properties e.g. {{device.name}},

• {{devices}} – children of current device or group, you can access their properties e.g. {{devices[14200].name}},

• {{devicesCount}} – number of devices,

• {{deviceGroupsCount}} – number of groups,

• {{usersCount}} – number of users,

• {{criticalDevicesCount}} – number of devices with critical alarm status,

• {{onlineCount}} – number of online devices.

We propose to add the following new variables, to provide context for a server-side generator of HTML content:

• {{widgetHeight}} - the height in pixels of the usable area of the widget

• {{widgetWidth}} -the width in pixels of the usable area of the widget

• {{widgetTheme}} - the theme selected for the widget, in the "Appearance" configuration page.

• {{widgetHeaderStyle}} - the header style selected for the widget in teh "Appearance" configuration page.