Fileman default open and Screenshot viewer (#1102)

* WIP expensive way

* Add default file handling to fileman

* Remove screenshot_reader and tests

* Read data in chunks

* Format

* Fix error text position

* PR feedback

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Co-authored-by: kallanreed <kallanreed@outlook.com>

firmware/application/apps/ui_fileman.hpp

@@ -213,6 +213,8 @@ class FileManagerView : public FileManBaseView {
     void on_new_dir();
     void on_new_file();
 
+    bool handle_file_open();
+
     // True if the selected entry is a real file item.
     bool selected_is_valid() const;