some improvements for developing plugins

doc/test-plugin.qdoc

@@ -0,0 +1,22 @@
+/*!
+    \page test-plugin.html
+    \title Testing your plugin
+    \brief This page describes how to test your plugin that you've just created
+
+    In order to make guhd load your plugin, you have 2 options. Either install it into the system or make guhd load it from your plugin build directory.
+
+    To install the plugin into the system, run
+    \code
+        $ make install
+    \endcode
+
+    To make guhd load the plugin from the plugins build directory, run guhd with an environment variable exported
+    \code
+        $ GUH_PLUGINS_DIR=/path/to/plugin/ guhd
+    \endcode
+
+    In order to easier debug things, it is advised to enable debug output for the device manager and your plugin. E.g.
+    \code
+        $ GUH_PLUGINS_DIR=/path/to/plugin guhd -d DeviceManager,YourPlugin
+    \endcode
+*/

doc/write-plugins.qdoc

@@ -7,6 +7,7 @@
         \li \l{Getting started}
         \li \l{The plugin JSON File}
         \li \l{CreateMethods and SetupMethods}
+        \li \l{Testing your plugin}
         \li \l{Tutorials}
             \list
                 \li \l{Tutorial 1 - The "Minimal" plugin}

libguh/devicemanager.cpp

@@ -257,6 +257,10 @@ DeviceManager::~DeviceManager()
 QStringList DeviceManager::pluginSearchDirs()
 {
     QStringList searchDirs;
+    QByteArray envPath = qgetenv("GUH_PLUGINS_PATH");
+    if (!envPath.isEmpty()) {
+        searchDirs << envPath;
+    }
     searchDirs << QCoreApplication::applicationDirPath() + "/../lib/guh/plugins";
     searchDirs << QCoreApplication::applicationDirPath() + "/../plugins/";
     searchDirs << QCoreApplication::applicationDirPath() + "/../plugins/deviceplugins";
@@ -1025,7 +1029,7 @@ void DeviceManager::loadPlugins()
             loader.setLoadHints(QLibrary::ResolveAllSymbolsHint);
 
             if (!loader.load()) {
-                qCWarning(dcDeviceManager) << "Could not load plugin data of" << entry;
+                qCWarning(dcDeviceManager) << "Could not load plugin data of" << entry << "\n" << loader.errorString();
                 continue;
             }