Terminal panel updates

- Add TerminalSettings resource.
- Init/load terminal settings from terminal panel.
- Add terminal context menu (i.e. right-click PopupMenu).
- Add shortcut to open a new terminal Ctrl+Shift+T and make terminal panel
  visible.

addons/godot_xterm/editor_plugins/terminal/settings/default_copy_shortcut.tres

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+[gd_resource type="ShortCut" format=2]
+
+[resource]

addons/godot_xterm/editor_plugins/terminal/settings/default_new_terminal_shortcut.tres

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+[gd_resource type="ShortCut" load_steps=2 format=2]
+
+[sub_resource type="InputEventKey" id=1]
+shift = true
+control = true
+command = true
+pressed = true
+scancode = 84
+
+[resource]
+shortcut = SubResource( 1 )

addons/godot_xterm/editor_plugins/terminal/settings/default_paste_shortcut.tres

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+[gd_resource type="ShortCut" format=2]
+
+[resource]

addons/godot_xterm/editor_plugins/terminal/settings/terminal_settings.gd

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+extends Resource
+
+enum FileType {
+	USE_SHELL_ENV,
+	CUSTOM_FILE,
+}
+
+enum CWDType {
+	USE_PROJECT_DIRECTORY,
+	CUSTOM_CWD,
+}
+
+### Shortcuts ###
+
+export (ShortCut) var new_terminal_shortcut = preload("./default_new_terminal_shortcut.tres")
+export (ShortCut) var copy_shortcut = preload("./default_copy_shortcut.tres")
+export (ShortCut) var paste_shortcut = preload("./default_paste_shortcut.tres")
+
+### Scroll settings ###
+
+# The maximum amount of lines the terminal keeps in its buffer.
+export var scrollback_buffer_lines := 1000
+# If true, mouse wheel up and down can be used to scroll the terminal.
+export var mouse_wheel_scroll := true
+# Whether or not to display scroll bar.
+export var show_scroll_bar := true
+
+# Copy/paste settings.
+export var copy_on_selection := false
+
+# Font settings.
+export var font_size: int = 14
+export var letter_spacing: int = 0
+export var line_height: float = 1.2
+export var ctrl_scroll_to_resize_font := true
+
+# Bell settings.
+export var visual_bell := true
+export var audio_bell := true
+export var bell_sound: AudioStream = preload("../../../themes/audio/bell.wav")
+
+# Exec args.
+export (FileType) var file_type := FileType.USE_SHELL_ENV
+export var custom_file := "/bin/sh"
+
+export (CWDType) var cwd_type := CWDType.USE_PROJECT_DIRECTORY
+export var custom_cwd := ""
+
+export var args := PoolStringArray()
+
+export var use_os_env := true
+export var extra_env := {
+	TERM = "xterm-256color",
+	COLORTERM = "truecolor",
+}
+
+
+func _init(p_copy_on_selection := false):
+	copy_on_selection = p_copy_on_selection