Allow drag applications in menu plugin so they can be dropped somewhere.

The only supported case seems to be drop them onto pcmanfm/desktop now.
Add a callback into plugin init struct for panel context menu update.

This way plugin instance can add custom menu items into context menu
without manual creation of menu and hacking it later.
Replace all the deprecated APIs with used in GTK 2.24.x.

Also raise min GTK+ version to 1.18 - that is required by LibFM anyway.
Disable direct access to GTK+ classes data (see GSEAL_ENABLE macro).
Do make in src/plugins/cpu a bit simpler, include <lxpanel/plugin.h> was overkill.
Fix make dist target, it will not compile successfully.
Fix crash on adding new panel.
Optimize lxpanel_plugin_popup_set_position_helper() parameters.
Place plugin configuration window near the plugin, not at bottom of screen.

We can use lxpanel_plugin_popup_set_position_helper() then move the window.
Dragging of taskbar buttons
Implement 'pager' configuration using patch from Lubuntu.

Only Openbox and Compiz WMs are supported now. For other WMs the context
menu option 'Desktop Pager Settings' will be disabled as it was before.
For Openbox it requires 'obconf' and for Compiz - either 'ccsm' or 'simple-ccsm'.
If those commands aren't available then 'Desktop Pager Settings' will be disabled.
Explore /sys/class/hwmon/hwmon0 for sensors in 'thermal' plugin.

Decent systems often hane neither /proc/acpi/thermal_zone* nor
/sys/class/thermal/thermal_zone* to read readings but have the
/sys/class/hwmon/hwmon* sensors data instead.
[SF#327]Don't launch extra applications on double/triple click on launcher.

Launch button should check for GDK_BUTTON_PRESS type.
Remove include/lxpanel/plugin.h unused file.
Use human readable sensor names if available (like 'Core 0', etc.)
Simplify lxpanel_plugin_show_config_dialog() parameters - no Panel* is required.
Minor updates to .gitignore and weather plugin Makefile.am.
Updated weather plugin to use the new plugin API.
Initial config save impl.
Minor cleanup to conform to the new plugin API.