switchlayers plugin: Add 2 images for each category to switch on/off all layers of this category.

It contains:
- config.inc: include the other files in pmapper
- switchlayers.js: plugin code 
- switchlayers.css: styles for images

Dependancies:
- No dependancy

How to use:
- Enable the plugin by adding a line in config_XXXXX.xml file:
<pmapper>
    <ini>
        <pmapper>
....
            <plugins>switchlayers</plugins>
....
        </pmapper>
	</ini>
</pmapper>

- Set default plugin configuration by adding a line in config_XXXXX.xml file:
<pmapper>
   <ini>
....
        <pluginsConfig>
....        
            <switchlayers>
<!-- checks, small -->
<!--
                <srcImgOnUp>plugins/switchlayers/layerson-bw-12px.png</srcImgOnUp>
                <srcImgOff>plugins/switchlayers/layersoff-bw-12px.png</srcImgOff>
-->
<!-- colors, small -->
<!--
                <srcImgOnUp>plugins/switchlayers/layerson-12px.png</srcImgOnUp>
                <srcImgOff>plugins/switchlayers/layersoff-12px.png</srcImgOff>
-->
<!-- colors -->
<!--
                <srcImgOnUp>plugins/switchlayers/layerson2.png</srcImgOnUp>
                <srcImgOff>plugins/switchlayers/layersoff2.png</srcImgOff>
-->
                <srcImgOnUp>plugins/switchlayers/layerson3_up.png</srcImgOnUp>
                <srcImgOffUp>plugins/switchlayers/layersoff3_up.png</srcImgOffUp>
                <!-- insert after layers (0 / 1) ? -->
                <insertAfter>0</insertAfter>
                <!-- use different states for category (0 / 1 ) ? -->
                <useAllCatStates>1</useAllCatStates>
				<!-- if useAllCatStates is activated, other images are defined here -->
                <srcImgOnDown>plugins/switchlayers/layerson3_down.png</srcImgOnDown>
                <srcImgOffDown>plugins/switchlayers/layersoff3_down.png</srcImgOffDown>
                <srcImgOnMix>plugins/switchlayers/layerson3_mix.png</srcImgOnMix>
                <srcImgOffMix>plugins/switchlayers/layersoff3_mix.png</srcImgOffMix>
            </switchlayers>
....
        <pluginsConfig>
    </ini>
</pmapper>  

To do:
