clear filters when empty string is passed to applyFilter

boost::regex doesn't like empty strings in literal mode and throws exceptions.

src/browser.cpp

@@ -251,6 +251,12 @@ std::string Browser::currentFilter()
 
 void Browser::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		w.clearFilter();
+		w.clearFilterResults();
+		return;
+	}
 	try
 	{
 		w.showAll();

src/media_library.cpp

@@ -559,6 +559,25 @@ std::string MediaLibrary::currentFilter()
 
 void MediaLibrary::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		if (isActiveWindow(Tags))
+		{
+			Tags.clearFilter();
+			Tags.clearFilterResults();
+		}
+		else if (isActiveWindow(Albums))
+		{
+			Albums.clearFilter();
+			Albums.clearFilterResults();
+		}
+		else if (isActiveWindow(Songs))
+		{
+			Songs.clearFilter();
+			Songs.clearFilterResults();
+		}
+		return;
+	}
 	try
 	{
 		if (isActiveWindow(Tags))

src/playlist.cpp

@@ -142,6 +142,12 @@ std::string Playlist::currentFilter()
 
 void Playlist::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		w.clearFilter();
+		w.clearFilterResults();
+		return;
+	}
 	try
 	{
 		w.showAll();

src/playlist_editor.cpp

@@ -335,6 +335,20 @@ std::string PlaylistEditor::currentFilter()
 
 void PlaylistEditor::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		if (isActiveWindow(Playlists))
+		{
+			Playlists.clearFilter();
+			Playlists.clearFilterResults();
+		}
+		else if (isActiveWindow(Content))
+		{
+			Content.clearFilter();
+			Content.clearFilterResults();
+		}
+		return;
+	}
 	try
 	{
 		if (isActiveWindow(Playlists))

src/search_engine.cpp

@@ -264,6 +264,12 @@ std::string SearchEngine::currentFilter()
 
 void SearchEngine::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		w.clearFilter();
+		w.clearFilterResults();
+		return;
+	}
 	try
 	{
 		w.showAll();

src/tag_editor.cpp

@@ -740,6 +740,20 @@ std::string TagEditor::currentFilter()
 
 void TagEditor::applyFilter(const std::string &filter)
 {
+	if (filter.empty())
+	{
+		if (w == Dirs)
+		{
+			Dirs->clearFilter();
+			Dirs->clearFilterResults();
+		}
+		else if (w == Tags)
+		{
+			Tags->clearFilter();
+			Tags->clearFilterResults();
+		}
+		return;
+	}
 	try
 	{
 		if (w == Dirs)