[tui] Add EditorTab widget.

+ This adds support for tabbed editors wrapped by EditorTab widgets.
+ The Explorer widget now opens new EditorTabs when a file is selected
  with Enter.
+ The same file may not be opened multiple times.
+ Tabs can be switched with ALT+h or ALT+l (or ALT+ arrow keys)
+ Tabs cannot yet be closed :)

Fixes #9

src/tui/menu_bar.rs

@@ -1,4 +1,3 @@
-use crate::tui::component::Action::Pass;
 use crate::tui::component::{Action, Component, ComponentState};
 use crate::tui::menu_bar::MenuBarItemOption::{
     About, Exit, Reload, Save, ShowHideExplorer, ShowHideLogger,
@@ -33,12 +32,12 @@ enum MenuBarItemOption {
 impl MenuBarItemOption {
     fn id(&self) -> &str {
         match self {
-            MenuBarItemOption::Save => "Save",
-            MenuBarItemOption::Reload => "Reload",
-            MenuBarItemOption::Exit => "Exit",
-            MenuBarItemOption::ShowHideExplorer => "Show / hide explorer",
-            MenuBarItemOption::ShowHideLogger => "Show / hide logger",
-            MenuBarItemOption::About => "About",
+            Save => "Save",
+            Reload => "Reload",
+            Exit => "Exit",
+            ShowHideExplorer => "Show / hide explorer",
+            ShowHideLogger => "Show / hide logger",
+            About => "About",
         }
     }
 }
@@ -116,7 +115,7 @@ impl MenuBar {
         buf: &mut Buffer,
         opened: MenuBarItem,
     ) {
-        let popup_area = Self::rect_under_option(title_bar_anchor, area, 40, 10);
+        let popup_area = Self::rect_under_option(title_bar_anchor, area, 27, 10);
         Clear::default().render(popup_area, buf);
         let options = opened.options().iter().map(|i| ListItem::new(i.id()));
         StatefulWidget::render(