tell the user how to mute the WSL bell (fixes #413)

terminus-terminal/src/components/terminalSettingsTab.component.pug

@@ -29,7 +29,11 @@ h3.mt-3.mb-3 Terminal
                 [value]='"audible"'
             )
             | Audible
-
+            
+.alert.alert-info.d-flex.align-items-center(*ngIf='config.store.terminal.bell != "audible" && config.store.terminal.shell.startsWith("wsl")')
+    .mr-auto WSL terminal bell can only be muted via Volume Mixer
+    button.btn.btn-secondary((click)='openWSLVolumeMixer()') Show Mixer
+                
 .form-line
     .header
         .title Right click

terminus-terminal/src/components/terminalSettingsTab.component.ts

 import { Component } from '@angular/core'
-import { ConfigService } from 'terminus-core'
+import { ConfigService, ElectronService } from 'terminus-core'
+import { TerminalService } from '../services/terminal.service'
 
 @Component({
     template: require('./terminalSettingsTab.component.pug'),
@@ -7,5 +8,16 @@ import { ConfigService } from 'terminus-core'
 export class TerminalSettingsTabComponent {
     constructor (
         public config: ConfigService,
+        private electron: ElectronService,
+        private terminal: TerminalService,
     ) { }
+
+    openWSLVolumeMixer () {
+        this.electron.shell.openItem('sndvol.exe')
+        this.terminal.openTab({
+            id: '',
+            command: 'wsl.exe',
+            args: ['tput', 'bel'],
+        }, null, true)
+    }
 }