Add "Visual-Mode" (#192)

* Add "Visual-Mode"

This adds a "visual-mode" file selection feature where a range-selection follows the file-cursor.

Description of usage is added to the docs.

Also, the "normal" and the new "visual-mode-selection" are now preserved
when a `DirList` is reloaded.

Wrap-up of changes:
* Add command `toggle_visual`, mapped to `V`
* Add command `escape`, mapped to `ESCAPE`
* Add style `[visual_mode_selection]` for file entries which are
  "temporarily" selected by visual-mode
* For `JoshutoDirEntry`, the attribute `selected` has been renamed to
  `permanent_selected`, and a second selection-attribute
  `visual_mode_selected` has been added. "Setters" and "getters" have
  been adapted/added accordingly. The former "getter" for the `selecetd`
  attribute still exists and returns `True` for an entry which is
  "permanant selected" _or_ "visual-mode selected". So any higher logic
  which acts on selected files does not need to care about "how" and
  entry is selected.
* Each `JoshutoDirList` has an optional index of the file where
  visual-mode has been entered and re-calculates the "visual-mode
  selecetd" status for each entry any time the cursor-index changes.
* The footer has been extended so it shows a "VIS" marker when the user
  is in visual-mode.

This implementation of visual-mode is a bit different from the ranger
one, where the visual-selection is turned into a "normal selection" when
a command (like `copy-files`) is issued. This implementation keeps
both selections separate until the user toggles back to "normal mode".
Only then the visual-selection is taken over to the "normal selection".
The user also can withdraw the visual-selection with `escape`.

The `escape` command may be used also for other "reset"-actions in the
future.

* fix syntax for Rust stable

* cargo clippy

1 files changed, 1 insertions, 1 deletions

diff --git a/src/context/app_context.rs b/src/context/app_context.rs
index 364b5b1..66ffbc6 100644
--- a/src/context/app_context.rs
+++ b/src/context/app_context.rs
@@ -17,7 +17,7 @@ use crate::Args;
 use notify::{RecursiveMode, Watcher};
 use std::path;
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct UiContext {
     pub layout: Vec<Rect>,
 }